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Register a new userMeasuring molecular abundances in comet C/2014 Q2 (Lovejoy) using the APEX telescope
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Added by
Miguel de Val-Borro
Publication date
2017
fields
Physics
and research's language is
English
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Comet composition provides critical information on the chemical and physical processes that took place during the formation of the Solar system. We report here on millimetre spectroscopic observations of the long-period bright comet C/2014 Q2 (Lovejoy) using the Atacama Pathfinder Experiment (APEX) band 1 receiver between 2015 January UT 16.948 to 18.120, when the comet was at heliocentric distance of 1.30 AU and geocentric distance of 0.53 AU. Bright comets allow for sensitive observations of gaseous volatiles that sublimate in their coma. These observations allowed us to detect HCN, CH3OH (multiple transitions), H2CO and CO, and to measure precise molecular production rates. Additionally, sensitive upper limits were derived on the complex molecules acetaldehyde (CH3CHO) and formamide (NH2CHO) based on the average of the strongest lines in the targeted spectral range to improve the signal-to-noise ratio. Gas production rates are derived using a non-LTE molecular excitation calculation involving collisions with H2O and radiative pumping that becomes important in the outer coma due to solar radiation. We find a depletion of CO in C/2014 Q2 (Lovejoy) with a production rate relative to water of 2 per cent, and relatively low abundances of Q(HCN)/Q(H2O), 0.1 per cent, and Q(H2CO)/Q(H2O), 0.2 per cent. In contrast the CH3OH relative abundance Q(CH3OH)/Q(H2O), 2.2 per cent, is close to the mean value observed in other comets. The measured production rates are consistent with values derived for this object from other facilities at similar wavelengths taking into account the difference in the fields of view. Based on the observed mixing ratios of organic molecules in four bright comets including C/2014 Q2, we find some support for atom addition reactions on cold dust being the origin of some of the molecules.
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HCN J=1-0 emission from the long-period comet C/2013 R1 (Lovejoy) was observed from the Onsala Space Observatory on multiple occasions during the month before its perihelion passage on December 22, 2013. We report detections for seven different dates, spanning heliocentric distances (R_h) decreasing from 0.94 to 0.82 au. Estimated HCN production rates are generally higher than previously reported for the same time period, but the implied increase in production rate with heliocentric distance, Q_{HCN} proportionate to R_h^{-3.2}, represent well the overall documented increase since it was first observed at R_h=1.35. The implied mean HCN abundance relative to water in R1 Lovejoy is 0.2%. We also report on a detection of HCN with the new 3 mm receiver system at Onsala Space Observatory in comet C/2014 Q2 (Lovejoy) on January 14, 2015, when its heliocentric distance was 1.3 au. Relative to comet C/2013 R1 (Lovejoy), the HCN production rate of C/2014 Q2 (Lovejoy) was more than 5 times higher at similar heliocentric distances, and the implied HCN abundance relative to water 0.09%.
Observations of C/2014 Q2 (Lovejoy) comet were carried out on 7th February, 2015, at the observation station in Mayaki village (No. 583 -- Odesa-Mayaki observatory). The integrated-light photometry of the comet was conducted using RC-800 telescope (D = 80 cm; F = 214.0 cm) with FLI MicroLine 9000 CCD camera. The photometric primary reductions included dark-frame subtraction and flat-field correction. The photometric study of the comet plasma tails was performed using an interactive program to construct a series of longitudinal and transverse profiles of individual tail rays. The Shulman diffusion model was applied to interpret the calculated photometric profiles. The comparison of the experimental profiles and those calculated theoretically from the diffusion model enabled us to estimate the following physical parameters of the comet plasma tail: acceleration a = 176 m/sec$^2$ and lifetime of fluorescent ions $tau$ = 2.7$cdot$10$^3$ sec; longitudinal and transverse diffusion coefficients, and magnetic flux density B = 97 $pm$ 5.3 nT.
The apparition of bright comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy) in March-April 2013 and January 2015, combined with the improved observational capabilities of submillimeter facilities, offered an opportunity to carry out sensitive compositional and isotopic studies of the volatiles in their coma. We observed comet Lovejoy with the IRAM 30m telescope between 13 and 26 January 2015, and with the Odin submillimeter space observatory on 29 January - 3 February 2015. We detected 22 molecules and several isotopologues. The H$_2^{16}$O and H$_2^{18}$O production rates measured with Odin follow a periodic pattern with a period of 0.94 days and an amplitude of ~25%. The inferred isotope ratios in comet Lovejoy are $^{16}$O/$^{18}$O = 499 $pm$ 24 and D/H = 1.4 $pm$ 0.4 $times 10^{-4}$ in water, $^{32}$S/$^{34}$S = 24.7 $pm$ 3.5 in CS, all compatible with terrestrial values. The ratio $^{12}$C/$^{13}$C = 109 $pm$ 14 in HCN is marginally higher than terrestrial and $^{14}$N/$^{15}$N = 145 $pm$ 12 in HCN is half the Earth ratio. Several upper limits for D/H or 12C/13C in other molecules are reported. From our observation of HDO in comet C/2014 Q2 (Lovejoy), we report the first D/H ratio in an Oort Cloud comet that is not larger than the terrestrial value. On the other hand, the observation of the same HDO line in the other Oort-cloud comet, C/2012 F6 (Lemmon), suggests a D/H value four times higher. Given the previous measurements of D/H in cometary water, this illustrates that a diversity in the D/H ratio and in the chemical composition, is present even within the same dynamical group of comets, suggesting that current dynamical groups contain comets formed at very different places or times in the early solar system.
Long period comet C/2014 B1 (Schwartz) exhibits a remarkable optical appearance, like that of a discus or bi-convex lens viewed edgewise. Our measurements in the four years since discovery reveal a unique elongated dust coma whose orientation is stable with respect to the projected anti-solar and orbital directions. With no tail and no trail, the limited influence of radiation pressure on the dust coma sets a lower limit to the effective particle size of 0.1 mm, while the photometry reveals a peak coma scattering cross-section 27,000 sq km (geometric albedo 0.1 assumed). From the rate of brightening of the comet we infer a dust production rate of order 10 kg/s at 10 AU heliocentric distance, presumably due to the sublimation of supervolatile ices, and perhaps triggered by the crystallization of amorphous water ice. We consider several models for the origin of the peculiar morphology. The disk-like shape is best explained by equatorial ejection of particles from a nucleus whose spin vector lies near the plane of the sky. In this interpretation, the unique appearance of C/2014 B1 is a result of a near equality between the rotation-assisted nucleus escape speed (1 to 10 m/s for a 2 to 20 kilometer-scale nucleus) and the particle ejection velocity, combined with a near-equatorial viewing perspective. To date, most other comets have been studied at heliocentric distances less than half that of C/2014 B1, where their nucleus temperatures, gas fluxes and dust ejection speeds are much higher. The throttling role of nucleus gravity is correspondingly diminished, so that the disk morphology has not before been observed.
We aim to determine the production rates of several parent and product volatiles and the 12C/13C isotopic carbon ratio in the long-period comet C/2004 Q2 (Machholz), which is likely to originate from the Oort Cloud. The line emission from several molecules in the coma was measured with high signal-to-noise ratio in January 2005 at heliocentric distance of 1.2 AU by means of high-resolution spectroscopic observations using the Submillimeter Telescope (SMT). We have obtained production rates of several volatiles (CH3OH, HCN, H13CN, HNC, H2CO, CO and CS) by comparing the observed and simulated line-integrated intensities. Furthermore, multiline observations of the CH3OH (7-6) series allow us to estimate the rotational temperature using the rotation diagram technique. We find that the CH3OH population distribution of the levels sampled by these lines can be described by a rotational temperature of 40 pm 3 K. Derived mixing ratios relative to hydrogen cyanide are CO/CH3OH/H2CO/CS/HNC/H13CN/HCN = 30.9/24.6/4.8/0.57/0.031/0.013/1 assuming a pointing offset of 8 due to the uncertain ephemeris at the time of the observations and the telescope pointing error. The measured relative molecular abundances in C/2004 Q2 (Machholz) are between low- to typical values of those obtained in Oort Cloud comets, suggesting that it has visited the inner solar system previously and undergone thermal processing. The HNC/HCN abundance ratio of ~3.1% is comparable to that found in other comets, accounting for the dependence on the heliocentric distance, and could possibly be explained by ion-molecule chemical processes in the low-temperature atmosphere. From a tentative H13CN detection, the measured value of 97 pm 30 for the H12CN/H13CN isotopologue pair is consistent with a telluric value.
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